1. Field of the Invention
The present invention relates to the CDMA method (Code Division Multiple Access) used in radio systems and more exactly to allocation of a RAKE receiver when using polarization diversity.
2. Description of Related Art
A receiver operating on a RAKE principle comprises several branches, each of which may be synchronized with a different signal component at the same time. A RAKE receiver preferably operates as a diversity combiner of multipath propagated signals. RAKE receivers are used especially in CDMA receivers.
In a CDMA method, a narrow-band data signal of a user is multiplied by a spreading code having a considerably broader band than the data signal. Users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection between a base station and a mobile station, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of each user. The spreading codes are preferably selected in such a way that they correlate with each other as little as possible. The signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band, but they will appear as noise.
In prior art, especially in a base station of a radio system, space diversity is used between RAKE branches. This means that antennas of different diversity branches are placed at some distance from each other. The distance is generally several tens of wavelengths. The purpose of this is to make the correlation of the signals received by different antennas sufficiently small. Typically, correlation is at a rate of 0.7. As correlation is rather high, during fading, the signal of all the branches will be significantly reduced and in the worst case the signal is lost, irrespective of diversity and the connection is broken down.
The use of polarization diversity in RAKE receivers is also well known. In polarization diversity, RAKE branches receive a signal at different polarization levels. In a prior art solution, impulse response measurement is used for allocating RAKE branches that use polarization diversity. In that case, signals with the strongest impulse response are received, combined and detected at different RAKE branches. This method is best suited for a case in which the signal levels of diversity branches are equal. As this is not generally the case when using polarization diversity, this method is not optimal.